208v motors @240v

[txccider]

I have a 4 motor coffee roaster that the motors are rated 208v and I have 240v. After running the machine for a while the breakers in the panel shut the motors off. What would you suggest for a quick fix? There are 3 1/2 hp and 1 3/4hp motor in this mix...I think there is a 30 amp circuit. Please help!

 

[ScottyUK]

Ther are possibly more problems than you are aware of: the 208V motor sound slike a US specification, while 240V sounds like UK. US supplies are 60Hz, while UK are 50Hz. A motor is designed for a specific ratio of voltage and frequency. For 208V @ 60Hz the ratio is 3.46V/Hz. At 240V and 50Hz the ratio is 4.8V/Hz. The ratio is determined by the winding design and can't be easily changed. If you run at a V/Hz ratio greater than design, the motor draws a large magnetising current because the stator iron magnetically saturates. Even if you have 240V 60Hz the above will still apply, although the increase in V/Hz over design is smaller.

You really need to get the correct motor for the supply, although you could theoretically use a lower voltage supply and keep V/Hz correct IF the motor is not working flat out.





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If we learn from our mistakes,
I'm getting a great education!

 

[txccider]

Thanks Scotty,
The supply and motor are both US 60hz

The supply being 240v single phase and the motors 208v single phase.
So the ratio being 4.0 V/Hz instead of 3.46 V/Hz would be enough to trip the overload switches in the panel?
The motors seem to be running really hot.
Bob

 

[davidbeach]

If the motors are single voltage 208V (there are also dual voltage motors), their use at 240V, a 15% overvoltage, will cause overloading and overheating of the motor. Your best bet would be to get a buck/boost transformer connected in buck mode to drop the 240V down to something much closer to 208V.

 

[dpc]

I agree with davidbeach that motor is probably over-excited and operating in saturation.

I think you need to back up and get the motor full load amps from each nameplate and first make sure your basic circuit size and breaker size is adequate.

Then look into a small autotransformer to buck your voltage down to something closer to 208V. These are available from all major electrical equipment manufacturers, including GE, Cutler-Hammer (Eaton!) and SquareD (Groupe Schneider!!).

If the motors do not have built-in thermal protection, you should make sure they are adequately protected against overload by individual thermal overload devices. If you're feeding more than one motor from a panelboard breaker, the motors may not be adequately protected. But on the other hands, motors are designed to run HOT. The old electrician's rule was to hold your hand on the motor and see if you can count to 10 before you have to move your hand.

I could use some of that coffee this morning...

 

[txccider]

UPDATE,
As I wrote the original post in an agitated state at 3am, I went back to the drawing board and found the motors were 208/220, so I called up the roaster mfg and they said they set the overload switches low at the factory, and had me adjust them,up about 5%, now all is well.
Thank you all for your very kind posts!

 

[jraef]

I was just going to suggest that, honest I was...

Glad you got it worked out.

And what did we all learn today?

Don't puzzle on solving problems at 3AM. Every looks clearer after a good night's sleep.




"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[txccider]

AND A GOOD CUP OF COFFEE!
Bob

 

[dwftx34]

Just an fyi to scottyuk we have 3 basic voltages here in the states 480/277v 3-phase (4-wire 3-ungrounded
conductors and one grounded conductor) this transforms down to 208/120v 3-phase (4-wire 3-ungrounded
conductors and one grounded conductor). Then on our small commercial and residential areas we have 120/240v3-phase again (4-wire 3-ungrounded conductors and one grounded conductor. And 120/240 single phase with 2 ungrounded conductors and one grounded conductor.

 

[ScottyUK]

dwftx34,

Thanks for the note. I often forget that you have the 120/240 1-phase supply. The US seems to have an amazing lack of standardisation compared to Europe - and the US is one country!


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If we learn from our mistakes,
I'm getting a great education!

 

[jraef]

ScottyUK,
No the US is 50 separate sovereign states unified by a common constitution. Most of them came into existance at different times, using different standards, and often competing with one another. There are even more variations of voltages than what dfxtx34 mentioned, but somehow we manage.

We once had a monarchy who would have insisted that everything be compatible, but we got rid of it a long time ago...

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[ScottyUK]

jraef,

At risk of taking things off into the weeds, how much independence from central government do the individual states have?


----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[rbulsara]

scottyuk:

As for technical regulations and buildig code, each state is very inddependent in the USA, although they may rely on somewhat common standards, but each state issues its own professional license. Now its getting increasingly cumbersome to get licence bysimply reciporcity, that i sif you have professinal licence in one state, other would not just issue one, so easily.


As for laws for crime etc. goes, well there is no place to hide in the USA in general.

Also most of the electrical inventions and "standards" were developed or first applied in the USA, by GE and Westinghouse....although over time some became more common and well accepted standards, but any standards that were "invented" tries or tested remains in pockets or older installations in the USA. Where are other coutries who just followed had luxury of picking just the right 'standards' for themselves, so they have less clutter..

 

[bobferg]

What kind of roaster is it, if you don't mind me asking